cilium

There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile or primary cilia, which typically serve as sensory organelles. Along with flagella, they make up a group of organelles known as undulipodia.

Cilia may be "viewed as sensory cellularantennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation.

Types and distribution

Cilia are rare in plants, occurring most notably in cycads. Ciliates possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface. Some ciliates bare groups of cilia that are fused together into large mobile projections called cirri (singular, cirrus).

Larger eukaryotes, such as mammals, have motile cilia as well. Motile cilia are rarely found alone, usually present on a cell's surface in large numbers and beating in coordinated waves. In humans, for example, motile cilia are found in the lining of the trachea (windpipe), where they sweep mucus and dirt out of the lungs. In femalemammals, the beating of cilia in the Fallopian tubes moves the ovum from the ovary to the uterus.

In comparison to motile cilia, non-motile (or primary) cilia usually occur one per cell. The outer segment of the rod photoreceptor cell in the human eye is connected to its cell body with a specialized non-motile cilium. The dendritic knob of the olfactory neuron, where the odorant receptors are located, is also carrying non-motile cilia (about 10 cilia / dendritic knobs). Beyond these examples, almost all mammalian cells have a single non-motile primary cilium, and only recently has great progress been made in understanding the function of the primary cilium. Although the primary cilium was discovered in 1898, it has largely been ignored since then. Until the 1990s, the prevailing view of the primary cilium was that it was merely a vestigial organelle, without important function. Recent findings regarding its physiological roles in chemical sensation, signal transduction, and control of cell growth, have led scientists to acknowledge its importance in cell function, with the discovery of its role in diseases not previously recognized to involve the dysgenesis and dysfunction of cilia, such as polycystic kidney disease and congenital heart disease. The primary cilium is now known to play an important role in the function of many human organs.

Assembly and maintenance

To grow a cilium, the building blocks of the cilia such as tubulins and other partially assembled axonemal proteins are added to the ciliary tips which point away from the cell body. In most species bi-directional motility called intraflagellar transport or IFT plays an essential role to move these building materials from the cell body to the assembly site. IFT also carries the disassembled material to be recycled from the ciliary tip back to the cell body. By regulating the equilibrium between these two IFT processes, the length of cilia can be maintained dynamically.

Lack of functional cilia in mammalian Fallopian tubes can cause ectopic pregnancy. A fertilized ovum may not reach the uterus if the cilia are unable to move it there. In such a case, the ovum will implant in the Fallopian tubes, causing a tubal pregnancy, the most common form of ectopic pregnancy.

Since the flagellum of human sperm is actually a modified cilium, ciliary dysfunction can also be responsible for male infertility.

Of interest, there is an association of primary ciliary dyskinesia with left-right anatomic abnormalities such as situs inversus (a combination of findings known as Kartagener's syndrome) and other heterotaxic defects. These left-right anatomic abnormalities can also result in congenital heart disease. In fact, it has been shown that proper cilial function is responsible for the normal left-right asymmetry in mammals.